How to Resize image in Swift?

See my blog post, Resize image in swift and objective C, for further details.

Image resize function in swift as below.

func resizeImage(image: UIImage, targetSize: CGSize) -> UIImage? {
    let size = image.size
    
    let widthRatio  = targetSize.width  / size.width
    let heightRatio = targetSize.height / size.height
    
    // Figure out what our orientation is, and use that to form the rectangle
    var newSize: CGSize
    if(widthRatio > heightRatio) {
        newSize = CGSize(width: size.width * heightRatio, height: size.height * heightRatio)
    } else {
        newSize = CGSize(width: size.width * widthRatio, height: size.height * widthRatio)
    }
    
    // This is the rect that we've calculated out and this is what is actually used below
    let rect = CGRect(origin: .zero, size: newSize)
    
    // Actually do the resizing to the rect using the ImageContext stuff
    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    image.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    
    return newImage
}

Use the above function and resize image with 200*200 as below code

self.resizeImage(UIImage(named: "yourImageName")!, targetSize: CGSizeMake(200.0, 200.0))

swift3 updated

 func resizeImage(image: UIImage, targetSize: CGSize) -> UIImage {
    let size = image.size
    
    let widthRatio  = targetSize.width  / size.width
    let heightRatio = targetSize.height / size.height
    
    // Figure out what our orientation is, and use that to form the rectangle
    var newSize: CGSize
    if(widthRatio > heightRatio) {
        newSize = CGSize(width: size.width * heightRatio, height: size.height * heightRatio)
    } else {
        newSize = CGSize(width: size.width * widthRatio,  height: size.height * widthRatio)
    }
    
    // This is the rect that we've calculated out and this is what is actually used below
    let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)
    
    // Actually do the resizing to the rect using the ImageContext stuff
    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    image.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    
    return newImage!
}

---

For Swift 4.0 and iOS 10

    extension UIImage {
        func resizeImage(_ dimension: CGFloat, opaque: Bool, contentMode: UIViewContentMode = .scaleAspectFit) -> UIImage {
            var width: CGFloat
            var height: CGFloat
            var newImage: UIImage

            let size = self.size
            let aspectRatio =  size.width/size.height

            switch contentMode {
                case .scaleAspectFit:
                    if aspectRatio > 1 {                            // Landscape image
                        width = dimension
                        height = dimension / aspectRatio
                    } else {                                        // Portrait image
                        height = dimension
                        width = dimension * aspectRatio
                    }

            default:
                fatalError("UIIMage.resizeToFit(): FATAL: Unimplemented ContentMode")
            }

            if #available(iOS 10.0, *) {
                let renderFormat = UIGraphicsImageRendererFormat.default()
                renderFormat.opaque = opaque
                let renderer = UIGraphicsImageRenderer(size: CGSize(width: width, height: height), format: renderFormat)
                newImage = renderer.image {
                    (context) in
                    self.draw(in: CGRect(x: 0, y: 0, width: width, height: height))
                }
            } else {
                UIGraphicsBeginImageContextWithOptions(CGSize(width: width, height: height), opaque, 0)
                    self.draw(in: CGRect(x: 0, y: 0, width: width, height: height))
                    newImage = UIGraphicsGetImageFromCurrentImageContext()!
                UIGraphicsEndImageContext()
            }

            return newImage
        }
    }

---

Details

Xcode 10.2.1 (10E1001), Swift 5

Links

https://gist.github.com/eugenebokhan/5e62a0155754ae6aa6c3c13cf1744930

Image Resizing Techniques

Solution

import UIKit
import CoreGraphics
import Accelerate

extension UIImage {

    public enum ResizeFramework {
        case uikit, coreImage, coreGraphics, imageIO, accelerate
    }

    /// Resize image with ScaleAspectFit mode and given size.
    ///
    /// - Parameter dimension: width or length of the image output.
    /// - Parameter resizeFramework: Technique for image resizing: UIKit / CoreImage / CoreGraphics / ImageIO / Accelerate.
    /// - Returns: Resized image.

    func resizeWithScaleAspectFitMode(to dimension: CGFloat, resizeFramework: ResizeFramework = .coreGraphics) -> UIImage? {

        if max(size.width, size.height) <= dimension { return self }

        var newSize: CGSize!
        let aspectRatio = size.width/size.height

        if aspectRatio > 1 {
            // Landscape image
            newSize = CGSize(width: dimension, height: dimension / aspectRatio)
        } else {
            // Portrait image
            newSize = CGSize(width: dimension * aspectRatio, height: dimension)
        }

        return resize(to: newSize, with: resizeFramework)
    }

    /// Resize image from given size.
    ///
    /// - Parameter newSize: Size of the image output.
    /// - Parameter resizeFramework: Technique for image resizing: UIKit / CoreImage / CoreGraphics / ImageIO / Accelerate.
    /// - Returns: Resized image.
    public func resize(to newSize: CGSize, with resizeFramework: ResizeFramework = .coreGraphics) -> UIImage? {
        switch resizeFramework {
            case .uikit: return resizeWithUIKit(to: newSize)
            case .coreGraphics: return resizeWithCoreGraphics(to: newSize)
            case .coreImage: return resizeWithCoreImage(to: newSize)
            case .imageIO: return resizeWithImageIO(to: newSize)
            case .accelerate: return resizeWithAccelerate(to: newSize)
        }
    }

    // MARK: - UIKit

    /// Resize image from given size.
    ///
    /// - Parameter newSize: Size of the image output.
    /// - Returns: Resized image.
    private func resizeWithUIKit(to newSize: CGSize) -> UIImage? {
        UIGraphicsBeginImageContextWithOptions(newSize, true, 1.0)
        self.draw(in: CGRect(origin: .zero, size: newSize))
        defer { UIGraphicsEndImageContext() }
        return UIGraphicsGetImageFromCurrentImageContext()
    }

    // MARK: - CoreImage

    /// Resize CI image from given size.
    ///
    /// - Parameter newSize: Size of the image output.
    /// - Returns: Resized image.
    // https://developer.apple.com/library/archive/documentation/GraphicsImaging/Reference/CoreImageFilterReference/index.html
    private func resizeWithCoreImage(to newSize: CGSize) -> UIImage? {
        guard let cgImage = cgImage, let filter = CIFilter(name: "CILanczosScaleTransform") else { return nil }

        let ciImage = CIImage(cgImage: cgImage)
        let scale = (Double)(newSize.width) / (Double)(ciImage.extent.size.width)

        filter.setValue(ciImage, forKey: kCIInputImageKey)
        filter.setValue(NSNumber(value:scale), forKey: kCIInputScaleKey)
        filter.setValue(1.0, forKey: kCIInputAspectRatioKey)
        guard let outputImage = filter.value(forKey: kCIOutputImageKey) as? CIImage else { return nil }
        let context = CIContext(options: [.useSoftwareRenderer: false])
        guard let resultCGImage = context.createCGImage(outputImage, from: outputImage.extent) else { return nil }
        return UIImage(cgImage: resultCGImage)
    }

    // MARK: - CoreGraphics

    /// Resize image from given size.
    ///
    /// - Parameter newSize: Size of the image output.
    /// - Returns: Resized image.
    private func resizeWithCoreGraphics(to newSize: CGSize) -> UIImage? {
        guard let cgImage = cgImage, let colorSpace = cgImage.colorSpace else { return nil }

        let width = Int(newSize.width)
        let height = Int(newSize.height)
        let bitsPerComponent = cgImage.bitsPerComponent
        let bytesPerRow = cgImage.bytesPerRow
        let bitmapInfo = cgImage.bitmapInfo

        guard let context = CGContext(data: nil, width: width, height: height,
                                      bitsPerComponent: bitsPerComponent,
                                      bytesPerRow: bytesPerRow, space: colorSpace,
                                      bitmapInfo: bitmapInfo.rawValue) else { return nil }
        context.interpolationQuality = .high
        let rect = CGRect(origin: CGPoint.zero, size: newSize)
        context.draw(cgImage, in: rect)

        return context.makeImage().flatMap { UIImage(cgImage: $0) }
    }

    // MARK: - ImageIO

    /// Resize image from given size.
    ///
    /// - Parameter newSize: Size of the image output.
    /// - Returns: Resized image.
    private func resizeWithImageIO(to newSize: CGSize) -> UIImage? {
        var resultImage = self

        guard let data = jpegData(compressionQuality: 1.0) else { return resultImage }
        let imageCFData = NSData(data: data) as CFData
        let options = [
            kCGImageSourceCreateThumbnailWithTransform: true,
            kCGImageSourceCreateThumbnailFromImageAlways: true,
            kCGImageSourceThumbnailMaxPixelSize: max(newSize.width, newSize.height)
            ] as CFDictionary
        guard   let source = CGImageSourceCreateWithData(imageCFData, nil),
                let imageReference = CGImageSourceCreateThumbnailAtIndex(source, 0, options) else { return resultImage }
        resultImage = UIImage(cgImage: imageReference)

        return resultImage
    }

    // MARK: - Accelerate

    /// Resize image from given size.
    ///
    /// - Parameter newSize: Size of the image output.
    /// - Returns: Resized image.
    private func resizeWithAccelerate(to newSize: CGSize) -> UIImage? {
        var resultImage = self

        guard let cgImage = cgImage, let colorSpace = cgImage.colorSpace else { return nil }

        // create a source buffer
        var format = vImage_CGImageFormat(bitsPerComponent: numericCast(cgImage.bitsPerComponent),
                                          bitsPerPixel: numericCast(cgImage.bitsPerPixel),
                                          colorSpace: Unmanaged.passUnretained(colorSpace),
                                          bitmapInfo: cgImage.bitmapInfo,
                                          version: 0,
                                          decode: nil,
                                          renderingIntent: .absoluteColorimetric)
        var sourceBuffer = vImage_Buffer()
        defer {
            sourceBuffer.data.deallocate()
        }

        var error = vImageBuffer_InitWithCGImage(&sourceBuffer, &format, nil, cgImage, numericCast(kvImageNoFlags))
        guard error == kvImageNoError else { return resultImage }

        // create a destination buffer
        let destWidth = Int(newSize.width)
        let destHeight = Int(newSize.height)
        let bytesPerPixel = cgImage.bitsPerPixel
        let destBytesPerRow = destWidth * bytesPerPixel
        let destData = UnsafeMutablePointer<UInt8>.allocate(capacity: destHeight * destBytesPerRow)
        defer {
            destData.deallocate()
        }
        var destBuffer = vImage_Buffer(data: destData, height: vImagePixelCount(destHeight), width: vImagePixelCount(destWidth), rowBytes: destBytesPerRow)

        // scale the image
        error = vImageScale_ARGB8888(&sourceBuffer, &destBuffer, nil, numericCast(kvImageHighQualityResampling))
        guard error == kvImageNoError else { return resultImage }

        // create a CGImage from vImage_Buffer
        let destCGImage = vImageCreateCGImageFromBuffer(&destBuffer, &format, nil, nil, numericCast(kvImageNoFlags), &error)?.takeRetainedValue()
        guard error == kvImageNoError else { return resultImage }

        // create a UIImage
        if let scaledImage = destCGImage.flatMap({ UIImage(cgImage: $0) }) {
            resultImage = scaledImage
        }

        return resultImage
    }
}

Usage

Get image size

import UIKit

// https://stackoverflow.com/a/55765409/4488252
extension UIImage {
    func getFileSizeInfo(allowedUnits: ByteCountFormatter.Units = .useMB,
                         countStyle: ByteCountFormatter.CountStyle = .memory,
                         compressionQuality: CGFloat = 1.0) -> String? {
        // https://developer.apple.com/documentation/foundation/bytecountformatter
        let formatter = ByteCountFormatter()
        formatter.allowedUnits = allowedUnits
        formatter.countStyle = countStyle
        return getSizeInfo(formatter: formatter, compressionQuality: compressionQuality)
    }

    func getSizeInfo(formatter: ByteCountFormatter, compressionQuality: CGFloat = 1.0) -> String? {
        guard let imageData = jpegData(compressionQuality: compressionQuality) else { return nil }
        return formatter.string(fromByteCount: Int64(imageData.count))
    }
}

Test function

private func test() {
    guard let img = UIImage(named: "img") else { return }
    printInfo(of: img, title: "original image |")
    let dimension: CGFloat = 2000

    var framework: UIImage.ResizeFramework = .accelerate
    var startTime = Date()
    if let img = img.resizeWithScaleAspectFitMode(to: dimension, resizeFramework: framework) {
        printInfo(of: img, title: "resized image |", with: framework, startedTime: startTime)
    }

    framework = .coreGraphics
    startTime = Date()
    if let img = img.resizeWithScaleAspectFitMode(to: dimension, resizeFramework: framework) {
        printInfo(of: img, title: "resized image |", with: framework, startedTime: startTime)
    }

    framework = .coreImage
    startTime = Date()
    if let img = img.resizeWithScaleAspectFitMode(to: dimension, resizeFramework: framework) {
        printInfo(of: img, title: "resized image |", with: framework, startedTime: startTime)
    }

    framework = .imageIO
    startTime = Date()
    if let img = img.resizeWithScaleAspectFitMode(to: dimension, resizeFramework: framework) {
        printInfo(of: img, title: "resized image |", with: framework, startedTime: startTime)
    }

    framework = .uikit
    startTime = Date()
    if let img = img.resizeWithScaleAspectFitMode(to: dimension, resizeFramework: framework) {
        printInfo(of: img, title: "resized image |", with: framework, startedTime: startTime)
    }
}

private func printInfo(of image: UIImage, title: String, with resizeFramework: UIImage.ResizeFramework? = nil, startedTime: Date? = nil) {
    var description = "\(title) \(image.size)"
    if let startedTime = startedTime { description += ", execution time: \(Date().timeIntervalSince(startedTime))" }
    if let fileSize = image.getFileSizeInfo(compressionQuality: 0.9) { description += ", size: \(fileSize)" }
    if let resizeFramework = resizeFramework { description += ", framework: \(resizeFramework)" }
    print(description)
}

Output

original image | (5790.0, 8687.0), size: 17.1 MB
resized image | (1333.0, 2000.0), execution time: 0.8192930221557617, size: 1.1 MB, framework: accelerate
resized image | (1333.0, 2000.0), execution time: 0.44696998596191406, size: 1 MB, framework: coreGraphics
resized image | (1334.0, 2000.0), execution time: 54.172922015190125, size: 1.1 MB, framework: coreImage
resized image | (1333.0, 2000.0), execution time: 1.8765920400619507, size: 1.1 MB, framework: imageIO
resized image | (1334.0, 2000.0), execution time: 0.4638739824295044, size: 1 MB, framework: uikit

---

Since @KiritModi 's answer is from 2015, this is the Swift 3.0's version:

func resizeImage(image: UIImage, targetSize: CGSize) -> UIImage {
    let size = image.size

    let widthRatio  = targetSize.width  / image.size.width
    let heightRatio = targetSize.height / image.size.height

    // Figure out what our orientation is, and use that to form the rectangle
    var newSize: CGSize
    if(widthRatio > heightRatio) {
        newSize = CGSize(width: size.width * heightRatio, height: size.height * heightRatio)
    } else {
        newSize = CGSize(width: size.width * widthRatio,  height: size.height * widthRatio)
    }

    // This is the rect that we've calculated out and this is what is actually used below
    let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)

    // Actually do the resizing to the rect using the ImageContext stuff
    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    image.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return newImage!
}

---

For Swift 4 I would just make an extension on UIImage with referencing to self.

import UIKit
extension UIImage {
  func resizeImage(targetSize: CGSize) -> UIImage {
    let size = self.size
    let widthRatio  = targetSize.width  / size.width
    let heightRatio = targetSize.height / size.height
    let newSize = widthRatio > heightRatio ?  CGSize(width: size.width * heightRatio, height: size.height * heightRatio) : CGSize(width: size.width * widthRatio,  height: size.height * widthRatio)
    let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)

    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    self.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return newImage!
  }
}

---

For Swift 5.0 and iOS 12

extension UIImage {
    func imageResized(to size: CGSize) -> UIImage {
        return UIGraphicsImageRenderer(size: size).image { _ in
            draw(in: CGRect(origin: .zero, size: size))
        }
    }
}

use:

let image = #imageLiteral(resourceName: "ic_search")
cell!.search.image = image.imageResized(to: cell!.search.frame.size)

---

It's also possible to use AlamofireImage (https://github.com/Alamofire/AlamofireImage)

let size = CGSize(width: 30.0, height: 30.0)
let aspectScaledToFitImage = image.af_imageAspectScaled(toFit: size)

The function in the previous post gave me a blurry result.

---

Swift 3 Version and Extension style

This answer come from @Kirit Modi.

extension UIImage {

    func resizeImage(targetSize: CGSize) -> UIImage {
        let size = self.size

        let widthRatio  = targetSize.width  / size.width
        let heightRatio = targetSize.height / size.height

        // Figure out what our orientation is, and use that to form the rectangle
        var newSize: CGSize
        if(widthRatio > heightRatio) {
            newSize = CGSize(width: size.width * heightRatio, height: size.height * heightRatio)
        } else {
            newSize = CGSize(width: size.width * widthRatio,  height: size.height * widthRatio)
        }

        // This is the rect that we've calculated out and this is what is actually used below
        let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)

        // Actually do the resizing to the rect using the ImageContext stuff
        UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
        self.draw(in: rect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage!
    }
}

---

Updated Swift 5 version that uses the latest UIGraphicsImageRenderer API.

extension UIImage {
    public func resized(to target: CGSize) -> UIImage {
        let ratio = min(
            target.height / size.height, target.width / size.width
        )
        let new = CGSize(
            width: size.width * ratio, height: size.height * ratio
        )
        let renderer = UIGraphicsImageRenderer(size: new)
        return renderer.image { _ in
            self.draw(in: CGRect(origin: .zero, size: new))
        }
    }
}

---

Swift 4, extension version, NO WHITE LINE ON EDGES.

Nobody seems to be mentioning that if image.draw() is called with non-integer values, resulting image could show a white line artifact at the right or bottom edge.

extension UIImage {

    func scaled(with scale: CGFloat) -> UIImage? {
        // size has to be integer, otherwise it could get white lines
        let size = CGSize(width: floor(self.size.width * scale), height: floor(self.size.height * scale))
        UIGraphicsBeginImageContext(size)
        draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))
        let image = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return image
    }

---

Swift 4 Version

extension UIImage {
    func resizeImage(_ newSize: CGSize) -> UIImage? {
        func isSameSize(_ newSize: CGSize) -> Bool {
            return size == newSize
        }

        func scaleImage(_ newSize: CGSize) -> UIImage? {
            func getScaledRect(_ newSize: CGSize) -> CGRect {
                let ratio   = max(newSize.width / size.width, newSize.height / size.height)
                let width   = size.width * ratio
                let height  = size.height * ratio
                return CGRect(x: 0, y: 0, width: width, height: height)
            }

            func _scaleImage(_ scaledRect: CGRect) -> UIImage? {
                UIGraphicsBeginImageContextWithOptions(scaledRect.size, false, 0.0);
                draw(in: scaledRect)
                let image = UIGraphicsGetImageFromCurrentImageContext() ?? UIImage()
                UIGraphicsEndImageContext()
                return image
            }
            return _scaleImage(getScaledRect(newSize))
        }

        return isSameSize(newSize) ? self : scaleImage(newSize)!
    }
}

---

Swift 5 version respecting ratio (scaleToFill) and centering image:

extension UIImage {
    func resized(to newSize: CGSize) -> UIImage {
        return UIGraphicsImageRenderer(size: newSize).image { _ in
            let hScale = newSize.height / size.height
            let vScale = newSize.width / size.width
            let scale = max(hScale, vScale) // scaleToFill
            let resizeSize = CGSize(width: size.width*scale, height: size.height*scale)
            var middle = CGPoint.zero
            if resizeSize.width > newSize.width {
                middle.x -= (resizeSize.width-newSize.width)/2.0
            }
            if resizeSize.height > newSize.height {
                middle.y -= (resizeSize.height-newSize.height)/2.0
            }
            
            draw(in: CGRect(origin: middle, size: resizeSize))
        }
    }
}

---

UIImage Extension Swift 5

extension UIImage {
    func resize(_ width: CGFloat, _ height:CGFloat) -> UIImage? {
        let widthRatio  = width / size.width
        let heightRatio = height / size.height
        let ratio = widthRatio > heightRatio ? heightRatio : widthRatio
        let newSize = CGSize(width: size.width * ratio, height: size.height * ratio)
        let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)
        UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
        self.draw(in: rect)
        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()
        return newImage
    }
}

Use : UIImage().resize(200, 300)

---

All of the listed answers so far seem to result in an image of a reduced size, however the size isn't measured in pixels. Here's a Swift 5, pixel-based resize.

extension UIImage {
    func resize(_ max_size: CGFloat) -> UIImage {
        // adjust for device pixel density
        let max_size_pixels = max_size / UIScreen.main.scale
        // work out aspect ratio
        let aspectRatio =  size.width/size.height
        // variables for storing calculated data
        var width: CGFloat
        var height: CGFloat
        var newImage: UIImage
        if aspectRatio > 1 {
            // landscape
            width = max_size_pixels
            height = max_size_pixels / aspectRatio
        } else {
            // portrait
            height = max_size_pixels
            width = max_size_pixels * aspectRatio
        }
        // create an image renderer of the correct size
        let renderer = UIGraphicsImageRenderer(size: CGSize(width: width, height: height), format: UIGraphicsImageRendererFormat.default())
        // render the image
        newImage = renderer.image {
            (context) in
            self.draw(in: CGRect(x: 0, y: 0, width: width, height: height))
        }
        // return the image
        return newImage
    }
} 

Usage:

image.resize(500)

---

Swift 4 Solution-

Use this function

func image(with image: UIImage, scaledTo newSize: CGSize) -> UIImage {
    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    image.draw(in: CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height))
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()
    drawingImageView.image = newImage
    return newImage ?? UIImage()
}

Calling a function:-

image(with: predictionImage, scaledTo: CGSize(width: 28.0, height: 28.0)

here 28.0 is the pixel size that you want to set

---

Swift 4.2 version of @KiritModi answer

func resizeImage(image: UIImage, targetSize: CGSize) -> UIImage {
    let size = image.size

    let widthRatio  = targetSize.width  / size.width
    let heightRatio = targetSize.height / size.height

    var newSize: CGSize
    if(widthRatio > heightRatio) {
        newSize = CGSize(width: size.width * heightRatio, height: size.height * heightRatio)
    } else {
        newSize = CGSize(width: size.width * widthRatio, height: size.height *      widthRatio)
    }

    let rect = CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height)

    UIGraphicsBeginImageContextWithOptions(newSize, false, 1.0)
    image.draw(in: rect)
    let newImage = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return newImage!
}

---

Here's a general method (in Swift 5) for downscaling an image to fit a size. The resulting image can have the same aspect ratio as the original, or it can be the target size with the original image centered in it. If the image is smaller than the target size, it is not resized.

extension UIImage {
    func scaledDown(into size:CGSize, centered:Bool = false) -> UIImage {
        var (targetWidth, targetHeight) = (self.size.width, self.size.height)
        var (scaleW, scaleH) = (1 as CGFloat, 1 as CGFloat)
        if targetWidth > size.width {
            scaleW = size.width/targetWidth
        }
        if targetHeight > size.height {
            scaleH = size.height/targetHeight
        }
        let scale = min(scaleW,scaleH)
        targetWidth *= scale; targetHeight *= scale
        let sz = CGSize(width:targetWidth, height:targetHeight)
        if !centered {
            return UIGraphicsImageRenderer(size:sz).image { _ in
                self.draw(in:CGRect(origin:.zero, size:sz))
            }
        }
        let x = (size.width - targetWidth)/2
        let y = (size.height - targetHeight)/2
        let origin = CGPoint(x:x,y:y)
        return UIGraphicsImageRenderer(size:size).image { _ in
            self.draw(in:CGRect(origin:origin, size:sz))
        }
    }
}

---

calling of resizeimage method

let image1 = resizeimage(image: myimage.image!, withSize: CGSize(width:200, height: 200))

method for resizeing image

func resizeimage(image:UIImage,withSize:CGSize) -> UIImage {
        var actualHeight:CGFloat = image.size.height
        var actualWidth:CGFloat = image.size.width
        let maxHeight:CGFloat = withSize.height
        let maxWidth:CGFloat = withSize.width
        var imgRatio:CGFloat = actualWidth/actualHeight
        let maxRatio:CGFloat = maxWidth/maxHeight
        let compressionQuality = 0.5
        if (actualHeight>maxHeight||actualWidth>maxWidth) {
            if (imgRatio<maxRatio){
                //adjust width according to maxHeight
                imgRatio = maxHeight/actualHeight
                actualWidth = imgRatio * actualWidth
                actualHeight = maxHeight
            }else if(imgRatio>maxRatio){
                // adjust height according to maxWidth
                imgRatio = maxWidth/actualWidth
                actualHeight = imgRatio * actualHeight
                actualWidth = maxWidth
            }else{
                actualHeight = maxHeight
                actualWidth = maxWidth
            }
        }
        let rec:CGRect = CGRect(x:0.0,y:0.0,width:actualWidth,height:actualHeight)
        UIGraphicsBeginImageContext(rec.size)
        image.draw(in: rec)
        let image:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
        let imageData = UIImageJPEGRepresentation(image, CGFloat(compressionQuality))
        UIGraphicsEndImageContext()
        let resizedimage = UIImage(data: imageData!)
        return resizedimage!
    }

---

Here you have two simple functions of UIImage extension:

func scaledWithMaxWidthOrHeightValue(value: CGFloat) -> UIImage? {

    let width = self.size.width
    let height = self.size.height

    let ratio = width/height

    var newWidth = value
    var newHeight = value

    if ratio > 1 {
        newWidth = width * (newHeight/height)
    } else {
        newHeight = height * (newWidth/width)
    }

    UIGraphicsBeginImageContextWithOptions(CGSize(width: newWidth, height: newHeight), false, 0)

    draw(in: CGRect(x: 0, y: 0, width: newWidth, height: newHeight))

    let image = UIGraphicsGetImageFromCurrentImageContext()
    UIGraphicsEndImageContext()

    return image
}

func scaled(withScale scale: CGFloat) -> UIImage? {

    let size = CGSize(width: self.size.width * scale, height: self.size.height * scale)

    UIGraphicsBeginImageContextWithOptions(size, false, 0)

    draw(in: CGRect(x: 0, y: 0, width: size.width, height: size.height))

    let image = UIGraphicsGetImageFromCurrentImageContext()

    UIGraphicsEndImageContext()

    return image
}

---

Example is for image minimize to 1024 and less

func resizeImage(image: UIImage) -> UIImage {

    if image.size.height >= 1024 && image.size.width >= 1024 {

        UIGraphicsBeginImageContext(CGSize(width:1024, height:1024))
        image.draw(in: CGRect(x:0, y:0, width:1024, height:1024))

        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage!

    }
    else if image.size.height >= 1024 && image.size.width < 1024
    {

        UIGraphicsBeginImageContext(CGSize(width:image.size.width, height:1024))
        image.draw(in: CGRect(x:0, y:0, width:image.size.width, height:1024))

        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage!

    }
    else if image.size.width >= 1024 && image.size.height < 1024
    {

        UIGraphicsBeginImageContext(CGSize(width:1024, height:image.size.height))
        image.draw(in: CGRect(x:0, y:0, width:1024, height:image.size.height))

        let newImage = UIGraphicsGetImageFromCurrentImageContext()
        UIGraphicsEndImageContext()

        return newImage!

    }
    else
    {
        return image
    }

}

---

You can use this for fit image at Swift 3;

extension UIImage {
    func resizedImage(newSize: CGSize) -> UIImage {
        // Guard newSize is different
        guard self.size != newSize else { return self }

        UIGraphicsBeginImageContextWithOptions(newSize, false, 0.0);
        self.draw(in: CGRect(x: 0, y: 0, width: newSize.width, height: newSize.height))
        let newImage: UIImage = UIGraphicsGetImageFromCurrentImageContext()!
        UIGraphicsEndImageContext()
        return newImage
    }

    func resizedImageWithinRect(rectSize: CGSize) -> UIImage {
        let widthFactor = size.width / rectSize.width
        let heightFactor = size.height / rectSize.height

        var resizeFactor = widthFactor
        if size.height > size.width {
            resizeFactor = heightFactor
        }

        let newSize = CGSize(width: size.width/resizeFactor, height: size.height/resizeFactor)
        let resized = resizedImage(newSize: newSize)
        return resized
    }
}

Usage;

let resizedImage = image.resizedImageWithinRect(rectSize: CGSize(width: 1900, height: 1900))

---

SWIFT 5 - XCODE 12 -- RESIZE IMAGE & No White line

I used a wonderful solution above for Swift 5. And I changed one bit to include the term "floor" as I was getting a white line around my resized images. This rounds it to the nearest pixel or something so it looks great! I also had to change the syntax around the image name when the function is called (last line).

//method for resizing image

func resizeimage(image:UIImage,withSize:CGSize) -> UIImage {
    var actualHeight:CGFloat = image.size.height
    var actualWidth:CGFloat = image.size.width
    let maxHeight:CGFloat = withSize.height
    let maxWidth:CGFloat = withSize.width
    var imgRatio:CGFloat = actualWidth/actualHeight
    let maxRatio:CGFloat = maxWidth/maxHeight
    let compressionQuality = 0.5
    if (actualHeight>maxHeight||actualWidth>maxWidth) {
        if (imgRatio<maxRatio){
            //adjust width according to maxHeight
            imgRatio = maxHeight/actualHeight
            actualWidth = floor(imgRatio * actualWidth)
            actualHeight = maxHeight
        }else if(imgRatio>maxRatio){
            // adjust height according to maxWidth
            imgRatio = maxWidth/actualWidth
            actualHeight = imgRatio * actualHeight
            actualWidth = maxWidth
        }else{
            actualHeight = maxHeight
            actualWidth = maxWidth
        }
    }
    let rec:CGRect = CGRect(x:0.0,y:0.0,width:actualWidth,height:actualHeight)
    UIGraphicsBeginImageContext(rec.size)
    image.draw(in: rec)
    let image:UIImage = UIGraphicsGetImageFromCurrentImageContext()!
    let imageData = UIImageJPEGRepresentation(image, CGFloat(compressionQuality))
    UIGraphicsEndImageContext()
    let resizedimage = UIImage(data: imageData!)
    return resizedimage!
}

//calling of resizeimage method:

let myimage = UIImage(named: "imagename")

let image1 = resizeimage(image: myimage!, withSize: CGSize(width:50, height: 50)).withRenderingMode(.alwaysOriginal)

---